How to read HRM-10 Internal Resistance Tester

[bobcat2000]

I bought a HRM-10 Internal Resistance Tester.  Sadly, I don't know how to read it.  As least I know I need to open the clamps so they are not touching each others before I can get the readings.

If there is not much trouble, would you guys teach me how to read the numbers please?

How do I know if the battery is good or bad from reading those 2 numbers?
Do I fully charge the battery first and then get the readings from the HRM-10?
Run the battery down and get another readings and compare the numbers before I can tell if the battery is good or bad?

Many Thanks!!!

https://www.fnirsi.com/products/hrm-10

[Paul T]

First, let me state that the FNIRSI HRM-10 is an excellent piece of test equipment.  The voltage and resistance values are super precise and super accurate. 

The purpose of such a "AC" tool is to evaluate the quality of new batteries coming down the assembly line.  If the electrolyte is poor (voltage out of range), or the metal attachments of wires are poor (internal resistance out of range), this meter will be able to reject those conditions.  It does so by comparing the test result to the established limits which can be input by the user.  If the reading is out of range, the part or lot of parts are flagged. 

The HRM-10 will report the open circuit voltage and the internal resistance of the cell.  It DOES NOT PLACE A LOAD on the cell.  The equivalent in a vehicle is measuring the torque of the engine and checking the slop in the drivetrain, but not actually applying horsepower to the wheels and driving it around.

Next, I must tell you that the readings of this unit are not very good at evaluating the state of health of the cells.   Meaning, as your cell ages, other ways of testing will produce more meaningful results.  You need a tester that "puts the cell on the dyno".  One such tester is the Opus BT-3400.  It will fully cycle a rechargeable cell, and also features a "quick test" which measures DC resistance UNDER LOAD.  Both these two values (mAh and mR) can be used to compare the new result to the result when the cell was new.  If you weren't tracking your rechargeable cell from new, you can instead compare your result to the exact product datasheet.   

Your questions lead me to believe you are testing rechargeables, so look to Opus or Maha for appropriate devices that include the types of cells you want to test. 

To answer your question about charging, the Opus manual recommends testing a fully charged cell, since it places a temporary DC load on the cell and measures how much the voltage drops.  A partially or fully discharged cell would provide skewed results.  If you are using the FNIRSI, I don't think it matters much the state of charge.  The AC impedance results should be similar either way.  I personally use a fully charged cell. I don’t think comparing internal resistance for different states of charge is meaningful.

If you happen to be testing primaries (not rechargeable) I would change my recommended tool.  First method is with a regular voltmeter and resistor, or using just a meter with an appropriate battery test function.  Do an open circuit voltage test first, note the value.  Then apply a load which is 1/10 of the mA*h rating of the cell (for instance 15 mA for a 150 mA*h 9V rectangle cell).  Observe how much the voltage reading changes.  If it goes from 9.5V to 8.5V, the cell is probably like new.  If it goes from 8.5V to 6.5V, the cell is nearly expired.

If you want to do a full discharge of a primary cell, which is of course destructive, you'll want a tool that reports mW*h or W*h. The "W" signifies is power, similar to horsepower in a car.  You want to avoid "mA*h" or "A*h" test, which is similar to measuring RPM of an engine.  Unfortunately, common tools designed for rechargeable cells are being misused by YouTubers to evaluate primary cells and they wonder why they get strange results.  The best assessment of the quality of a primary cell is to measure the total power the cell can provide.  Power in Watts is Volts times Amperes, not Amperes alone.  Very important for accurate assessments!  Comparing mAh measurements across different cell chemistries contains a great deal of error.  Comparing across different brands of the same chemistry also will carry some error, but not as much as comparing lithium to alkaline and so on.

The HRM-10 can be used to measure the open circuit voltage.  With experience, this can be useful for determining the POSSIBLE capacity remaining in a cell.  Then again, any volt meter can do this.  The internal resistance check will only tell you whether or not the cell is made to high quality standards.  The lower the internal resistance, the better it is constructed.   If you get a zero voltage reading and a zero resistance reading, the cell is shorted.

Hopefully nobody is buying this unit thinking they can check the state of health of their batteries around the house and in their cars.  It can't really do that, other tools are much better.  However, if you are building battery packs from raw cells, this device can help match up similar quality cells so that they discharge at the same rate.   The device can also identify bad cells in a poorly performing battery pack.  The cell with low voltage and/or high resistance is likely to problem cell.

[bobcat2000]

Thanks for you input.

So, this HRM-10 is not really a "tester".
It doesn't test nothing since you can't tell if the battery is good or bad by reading that 2 numbers.

It is more like a "matcher".
If you "rate" a bunch of batteries that give you the same 2 numbers, you can tell the batteries may have the same "performance".  But if you really need to make sure that the batteries can do what they claim they can do, you will have to "test" them which the HRM-10 cannot do.

Am I reading you correctly?

[Paul T]

My assessment is the HRM-10 will confirm the cell can stand up on its feet, but it won’t tell you how fast it can run or how far it can go.  This has more to do with the test method, not the units of measure.

I did some testing comparing internal resistance using both AC conductance and DC loading. The AC method only slightly correlated with how well the cell could still hold a charge. The relationship with DC IR was much stronger because it reveals a significant aging factor. See battery universe article 802a.

https://batteryuniversity.com/article/bu-802a-how-does-rising-internal-resistance-affect-performance

[Paul T]

If you do use the Opus tester, here's some information to help you determine the health of your cells.

If you do a full cycle, the mAh on the screen reports how well it delivered Amperes before reaching the terminal voltage (for this device, 3.1V).  There is a graph below, not sure what equipment is used.  Typically takes a few hours.

To determine the health, you can divide this result by the mAh rating on the spec sheet from the battery manufacturer.  The resulting number is a general % of the health left in the battery.

Then, activate a "quick test" or "internal resistance test" which typically takes a few seconds.  This will give you a milliohm reading (mR).  Not sure you can directly compare this against the specification sheet, it is very device dependent.  All you can do is compare this against other the exact same cell brand and part number.

The following chart was found on the internet for 18650 cells. I embellished it with colored boxes and trend lines, for a potential "binning" of cells based on the Opus quick-test.  The person making the chart basically took the two numbers I described above across a large number of cells of different brands and plotted the points only.

The "correction" of dividing by the spec sheet mAh rating to get a % was key to getting a chart that shows a strong relationship.  This is important to note, because I stated before a pure current accumulation test ignores voltage - and voltage discharge curves can be very different from brand to brand and part number to part number.

Reminder: These results include the voltage depression due to being loaded, and will produce a HIGHER calculated resistance result than the AC conduction method of the FNIRSI.  For those lower results, you probably won't be able to correlate with the cell's remaining health.

[bobcat2000]

Thanks for the input again.

Bottom line.
Lower numbers are better than higher numbers.  Does not matter what I use to "test" the batteries (use my tongue to test a 9v battery for example).

Come to think about this, I don't think I need to use anything to test the batteries to tell the health of them.  When the run-down batteries are charged either very quickly or very long, I know the batteries are bad.

Back to the HRM-10.
The HRM-10 is not reporting the actual IR since it is not load-testing the battery.  The manual says that it is pumping a 1k pulse to the battery.  So, I guess it is counting pulses or timing to estimate the IR (or the K value).  I know what R is. I still don't know what K is.

[Paul T]

They are using a 1 kilohertz (1kHz) AC sine wave to test the impedance (resistance). The K is simply a prefix.

[Phil1977]

These testers can not judge the state of health of a battery without further testing or comparison.

But they are really useful in failure analysis. If a notebook battery e.g. has 6 cells and one has an internal resistance that is 3 times higher than the rest while it´s voltage is 0.5V lower then you directly know it´s bad.

And beside testing batteries, this thing is also good for testing cables, switches, capacitors, contacts in general. I once have build a cell pack from cylindrical LFP-cells with screw terminals. Checking the resistance from the cell terminal to the bus-bar with the resistance meter showed me the one contact with insulating residues way before anything else.

It´s hard to give unambiguous good/bad thresholds for this. But after gathering some real world experiences you will appreciate the indications you get with this thing.

PS: One word of caution: It´s a great and important feature that it uses 4-wire measurement. But in my experience the resistance meter does not reliably recognize if one of the four wires is not correctly contacting - and this leads to unplausible low or high measurement values. For this reason I have build some pseudo-4W-probes which are reliably shorted shortly before their single tips. Of course they have 10-25mOhm of own resistance, but for me the rate of "first false alarms" is much lower if I test with these cables.

[bobcat2000]

They are using a 1 kilohertz (1kHz) AC sine wave to test the impedance (resistance). The K is simply a prefix.

I know what K stand for.  I don't know what impedance is or what it does.

[bobcat2000]

...For this reason I have build some pseudo-4W-probes which are reliably shorted shortly before their single tips. Of course they have 10-25mOhm of own resistance, but for me the rate of "first false alarms" is much lower if I test with these cables.

Isn't this what this HRM-10 is for?  Their site kinda says that the resistance of the wires will not affect the reading.

[Phil1977]

...For this reason I have build some pseudo-4W-probes which are reliably shorted shortly before their single tips. Of course they have 10-25mOhm of own resistance, but for me the rate of "first false alarms" is much lower if I test with these cables.

Isn't this what this HRM-10 is for?  Their site kinda says that the resistance of the wires will not affect the reading.

Yes, exactly, 4-wire measurements are not affected by the cabling resistance. But if you replace the original cabling by probes which only have two instead of four tips, then you decrease the chance of bad contacting but add a little bit of resistance offset.

[Paul T]

I know what K stand for.  I don't know what impedance is or what it does.

Impedance is the vector sum of resistance and reactance when an AC signal is applied. For cells, the reactance portion is small and can essentially be ignored. So “resistance” and “impedance” can be used interchangeably for most cell test purposes.  That makes things easy. 

The AC method enables the DC resistance measurement without presenting the DC load.

It’s impossible to determine internal resistance without a dynamic measurement. An AC signal makes very small changes and measures how the cell reacts. For the DC load method, the dynamic measurement is comparing two different external load conditions.  The smaller the load is, the closer the results of the two methods will be.

The reason a DC internal resistance provides correlation to cell aging is the effect of loading the cell.  A cell that cannot keep up with the current demand due to aging will lose more voltage.  This loss of voltage portion of the DC evaluation is reported as part of the total internal resistance. 

In summary, you can look at it this way

Impedance ~= Resistance

Internal Resistance = No-Load Resistance + Load-Induced Resistance

…where load-induced resistance is dependent on the severity of the DC load.

Resistance is the cause of power loss and heat in a cell during usage. Lower resistance is therefore very important attribute for a power source.

[bobcat2000]

Thanks for trying...

I still don't know what impedance is because I don't know what "vector sum" or "reactance" is.

I thought the HRM-10 was reporting the K reading that a K reading was much lower than a R reading from a load tester.

(e.g.) a load tester will give 100 Ohms.  The HRM-10 will give 50 Ohms from the same battery.

From what I can understand from you, it is because a load tester will heat up the battery.  This makes the reading higher.  The HRM-10 does not heat up the battery.  Therefore, the reading from the HRM-10 is lower.  So, there is nothing to do with resistance reading vs impedance reading.

[Phil1977]

You can quite easily visualize why the AC-test gives lower readings than the load-test:

Each battery cell has a physical capacitance (like a capacitor, just by having charged plates next to each other) and a chemical capacitance that provides >99.9% of the stored energy. It´s equivalent circuit would be a battery in parallel with a cap.

A load tester does a slow measurement so that the cap does not contribute to the result. The HRM-10 or equivalent AC-testers can't distinguish, with 1kHz of AC frequency both components are significant.

Does that mean the AC-tested value is useless? No, definitely not. Its just another type of test. And also the resistance value of the load tester is always a compromise because the model with one physical and one chemical capacitance is extremely simplified. The chemical capacitance can only be fully described by a multidimensional charge/discharge curve, it´s much too complex for one value. Professional battery testers can record lots of impedance/frequency curves over a huge range of parameters like SOC, temperature, age, ...

[bobcat2000]

I think I understand what this guy is saying.

https://www.electronicdesign.com/technologies/test-measurement/article/21246713/keysight-technologies-measuring-acir-of-lithium-ion-cells

The HRM-10 is doing a ACIR measurement.  The reason the HRM-10 gives a lower reading is because it filters out parts of the battery.

[Paul T]

That is a good article.  Also, poke around Battery University to learn things simply and quickly.

Here's another site describing the DC method.  There is also a CIT method.  More overview is available here.

The HRM tests 100% AC, which on average is 0V DC.  It does not DC load the cell, which causes additional resistance under load to arise.  I don't think the additional resistance comes from heating, just the rising inefficiency of the electrolyte and plate electro-chemical performance.

The heat generally comes usage at higher current.  It's a result of the higher internal resistance under load, not the other way around.

If you run a LED flashlight on good lithium cells, the head where the emitter is gets hot, the batteries stay generally cool.  If you use older lithium cells, their DC resistance under load is now higher, and they create more heat.  After a while running the flashlight, both the grip and the LED end will get hot.  This accelerates further decline of the cells and reduces the LED’s ability to stay cool.   

This is why you would want to discard cells which have developed a high internal impedance. Some chargers like the Maha will do a pre-test and refuse to charge a cell and have a IR that is too high.  The heat they create could cause an explosion or fire or incorrect charge.

[Paul T]

I just found a good article from Energizer on internal resistance.  It's a short read.

https://data.energizer.com/pdfs/batteryir.pdf

Also this document

https://data.energizer.com/pdfs/alkaline_appman.pdf

Their formula uses different terms than I did, but the same really.

Total Effective Resistance = Electrical Resistance + Ionic Resistance

And they go on to say "the 1000 Hz impedance value will be less than the total effective resistance value for the same battery."

[bobcat2000]

Correct.  The HRM-10 is only reading the value from Electrical Resistance.  It is not reading the value from the Ionic Resistance.  That's why the reading is lower.